I gave a try at changing Account Control Level with no success. Here are the setup and setupui files. Hope it makes some sense to you, it sure doesn't to me!
setup
Trying to start process: C:\Users\WILLIA~1\AppData\Local\Temp\AffinitySetup\5e0335a4-f9c4-11ea-8eb6-34f39a6b759b\SetupUI.exe /log=true
Succeeded: 1
Waiting for process to finish...
Exit code: -1073740771
setupui
+Startup: 9/18/2020 11:33:59 AM
+OS version: 10.0.19041
+ProductName: Photo
+ProductType: Retail
+Opening HKLM...
+ OK
+Opening registry key: 'SOFTWARE\Serif\Affinity\Photo\1'...
+ OK
+Reading registry data...
+ Photo Install Path: [not found]
+ Photo Desktop shortcut: 1
+Getting ProgramW6432 environment variable...
+ OK
+Install path not found, using default: C:\Program Files\Affinity\Photo
+Opening MSI database: Affinity.msi
+Opening database view: SELECT `Value` FROM `Property` WHERE `Property`='ProductVersion'
+ OK
+MsiViewExecute
+ OK
+MsiViewFetch
+ OK
+MsiRecordGetString
+ Value: 1.8.5.703
+Opening database view: SELECT `Value` FROM `Property` WHERE `Property`='ProductCode'
+ OK
+MsiViewExecute
+ OK
+MsiViewFetch
+ OK
+MsiRecordGetString
+ Value: {E0A227B9-8299-48C6-9FB2-71140FEF82B4}
+Install state: -1
+ IsThisVersionInstalled: False
+Opening database view: SELECT `Value` FROM `Property` WHERE `Property`='UpgradeCode'
+ OK
+MsiViewExecute
+ OK
+MsiViewFetch
+ OK
+MsiRecordGetString
+ Value: {21399BDD-4C2C-4565-9426-410BDC894DA7}
+MsiEnumRelatedProducts: 259
+ IsAnyVersionInstalled: False
+ProductCode: {E0A227B9-8299-48C6-9FB2-71140FEF82B4}
+ProductVersion: 1.8.5
+InstallType: Normal
+Initialising main window...
+ OK
+Main window loaded
+Unhandled exception:
+ Message: The image data generated an overflow during processing.
+ StackTrace:
at System.Windows.Media.Imaging.ColorConvertedBitmap.FinalizeCreation()
at System.Windows.Media.Imaging.ColorConvertedBitmap..ctor(BitmapSource source, ColorContext sourceColorContext, ColorContext destinationColorContext, PixelFormat format)
at System.Windows.Media.Imaging.BitmapImage.FinalizeCreation()
at System.Windows.Media.Imaging.BitmapImage.EndInit()
at System.Windows.Media.Imaging.BitmapImage..ctor(Uri uriSource, RequestCachePolicy uriCachePolicy)
at System.Windows.Media.Imaging.BitmapImage..ctor(Uri uriSource)
at SetupUI.MainWindow.UpdateImage()
at SetupUI.MainWindow.OnLoaded(Object sender, RoutedEventArgs e)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)
at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root)
at MS.Internal.LoadedOrUnloadedOperation.DoWork()
at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
at System.Windows.Media.MediaContext.Resize(ICompositionTarget resizedCompositionTarget)
at System.Windows.Interop.HwndTarget.OnResize()
at System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Interop.HwndSource.HwndTargetFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
+ Inner message: Overflow or underflow in the arithmetic operation.
+ Inner stackTrace:
